This invention relates to a method of measuring the amount of reduction of a dot film such as a screen negative or a screen positive, and a device of practicing the same.
The dot film is obtained by subjecting an original to halftone photography, and the dot film is printed on an orginal plate to form a latent image. Then, the original plate is subjected to development to provide a press plate. It has been known from experience that it is impossible to obtain a satisfactory press plate directly from a dot film subjected to halftone photography. Before such a satisfactory press plate is obtained, it is necessary to print the dot film on a proofreading original plate thereby to form a press plate and then to carry out a proof. According to the result of this proof, the dot percentage of the dot film is adjusted.
Thereafter, a proofreading press plate is formed again to perform a proof. If the result of the proof is acceptable, then the dot film is printed on a printing original plate. If the result of the proof is not acceptable, then the dot percentage of the film is further adjusted, and a proof is carried out again.
The adjustment of the dot percentage of the dot film is carried out by the process of so-called "reduction". In this process, the size of the dots is reduced by washing the dot film with an iron chelate group reducer. The adjustment of the dot percentage depends completely on a retoucher's intuition. In other words, he has known through experience a suitable extent of reduction (or decrease of a dot percentage) for a desired printing result, and he performs the adjustment according to this knowledge.
The amount of reduction can be objectively obtained through measurements done with a dot percentage meter before and after the reduction. Therefore, the amount of reduction can be obtained even by an unskilled person. Accordingly, if the reduction is performed while the amount of reduction specified by the skilled retoucher is being measured with the dot percentage meter, then the reduction can be achieved even by the unskilled person, which leads to an improvement of the efficiency of the process of reduction.
A conventional dot percentage meter operates to measure the quantity of light passed through the portion other than dots of a dot film, thereby to calculate the amount of reduction. In general, the reduced portion of a dot film changes yellow-brown. Light is applied to this faded portion and to a transparent portion of the dot film where no dot is provided, and the quantities of light passed through the two different portions are converted into electrical data from which the dot percentage is calculated.
The dots are as shown in FIG. 1(a), and they become as shown in FIG. 2(a) after reduction. In FIG. 2(a), the peripheral portion surrounding a dot 10 is a portion 12 subjected to reduction. The distribution of light transmissivity is affected by the reduction; more specifically, it is changed from the state shown in FIG. 1(b) to that shown in FIG. 2(b).
When the dot film is printed on an original plate or other films, the printing light can pass through the yellow-brown portion 12. Therefore, in measuring the dot percentage it is undesirable that the light transmissivity of the reduced portion 12 is varied.
An error in measurement due to the decrease of the dot percentage of the reduced portion may be prevented by employing a conventional method in which the shading of a dot film is converted into an electrical signal by means of, for instance, a vidicon to measure the area of a portion of the film where the density is higher than a predetermined value, thereby to measure the dot percentage.
However, a device for practicing this conventional method is bulky and expensive. In addition, even with the device, if the position of measurement before reduction is shifted from the position of measurement after reduction, then the amount of reduction measured includes a large amount of error. This error will be increased especially when a gradation portion is measured, due to the following reason: In the gradation portion, the size of a dot is different from the size of a dot adjacent thereto, and therefore the difference in light transmissivity due to the difference in size of dots is added to the difference in light transmissivity corresponding to the amount of reduction, when the position of measurement is changed.
Accordingly, in the conventional method it is necessary that the position of measurement before reduction strictly coincides with the position of measurement after reduction.